1. Field of the Invention
The present invention relates to a power cable which is optimum for long-distance and bulk power transmission, and particularly to a structure and a method of manufacturing for a power cable for DC submarine transmission and a submarine transmission line using such power cables.
2. Description of the Related Art
Conventionally, a solid cable (Mass-Impregnated Cable or Non-Draining Cable) using kraft paper as insulating tape material and impregnated with high-viscosity insulating oil (for example, 25 to 100 cst at 120xc2x0 C., and 500 to 2,000 cst at the maximum service temperature of the cable (50 to 60xc2x0 C.)) has been used as a long-distance and bulk power DC cable.
To attain a solid cable of a larger capacity, it will do to make the solid cable withstand a higher voltage and allow a larger current. A large current solid cable can be realized if a conductor having a sectional area as large as possible is used or the maximum service temperature of the conductor is made to be as high as possible. On the other hand, making the voltage of the cable high and making the service temperature high depend on the performance of an insulation. They cannot be realized unless a new technique is developed.
Recently, in order to transmit bulk power which has been impossible or difficult to be realized in a conventional solid cable with kraft paper insulation, a solid cable using polyolefin resin film as at least a part of insulating material is proposed. Investigation has been conducted on the proposed cable which can be used, for example, under a high voltage of DC 500 kV or higher, or at the conductor maximum service temperature of 60xc2x0 C. or higher (for example, around 80xc2x0 C.).
However, insulating oil used in this case is a high-viscosity insulating oil which has been used in a conventional solid cable. This is because the insulating oil of the cable impregnated in a factory has been thought to be necessary to avoid, along the whole cable line, uneven oil-distribution or oil-starvation caused by migration in order that the electric characteristic is prevented from deterioration in any condition. That is, particularly in the case of a long-distance submarine solid cable, the cable line is too long to feed or absorb insulating oil at its both ends. It has been therefore considered that only high-viscosity oil enough not to produce migration even at the maximum service temperature of the cable (usually 55xc2x0 C. or lower) can be used.
However, the following problems arise as conspicuous hurdles for making both service voltage and service temperature of the conventional solid cables high to ensure the large capacity thereof.
When current load is OFF after the conductor takes the maximum temperature in the state of load ON period, the temperature near the conductor drops down sharply so that the contraction of the insulating oil near the conductor is caused. Since the high-viscosity oil cannot move sufficiently rapidly from the outside of the insulating wall to the inside thereof, sometimes starvation of the insulating oil occurs near the conductor which may produce voids so that such voids are thereby likely to reduce the electric performance conspicuously.
That is, as the maximum service temperature of the conductor is attempted to make higher, (1) the treatment of the insulating oil becomes more difficult because the amount of the expansion and contraction of the insulating oil is increased, and (2) it becomes more necessary to take measures against the easiness of migration because of lowering of the viscosity of the insulating oil. In addition, the temperature at the time of load OFF drops down more sharply to thereby cause severe oil-starvation so that large voids are apt to be generated. Therefore, there is a problem that high electric stress cannot be applied to the cable insulation thoughtlessly.
Further, it has been tried to make an application of polyolefin resin film or a composite insulating tape of polyolefin resin film and kraft paper. However, in comparison with kraft paper consisting of porous natural wood pulp fiber, polyolefin resin film has no pores through which liquid can flow so that high-viscosity insulating oil are not allowed to pass. Therefore, when a cable core is impregnated with insulating oil in a factory with high-viscosity insulating oil, there arises a very serious situation that the impregnation of insulating oil becomes insufficient or impossible, or even if possible very hard to fully implement for an industrially reasonable process time, as the insulation layer is thicker. As a result, it could be hardly done to improve the industrial productivity or to increase the ratio of the polyolefin resin film in the composite insulating tape in order to achieve the expected purpose.
It is an object of the present invention to provide a solid cable in which the voltage can be made high and the service temperature thereof can be increased so that very large bulk power transmission can be realized.
It is another object of the present invention to provide a method of manufacturing such a solid cable and a transmission line using such solid cables.
The present invention is intended to solve the foregoing problems, and its characteristic is using medium-viscosity insulating oil as the insulating oil in a solid cable.
The term xe2x80x9cmedium-viscosity insulating oilxe2x80x9d herein means insulating oil the viscosity of which is not less than 10 centistokes (cst) and less than 500 centistokes (cst), at 60xc2x0 C. Particularly, the SP value (Solubility Parameter) of the insulating oil is preferably within a range of xc2x11.5 of the SP value of polyolefin resin film used in an insulation layer. Examples of the medium-viscosity insulating oil include polystyrene insulating oil, polybutene, mineral oil, synthetic oil mainly composed of alkylbenzene, heavy alkylate, or a mixture containing at least one of these oils. Particularly, it is preferable to contain dodecylbenzene (DDB).
It is preferable to use a tape containing polyolefin resin film as at least a part of an insulation layer of a cable according to the present invention. The tape containing polyolefin resin film includes a composite tape laminated with kraft paper on one side or both sides of polyolefin resin film, as well as an insulating tape consisting of polyolefin resin film singly. Particularly, it is preferable that a composite tape laminated with kraft paper on both sides of polyolefin resin film and an insulating tape consisting of polyolefin resin film singly are wound alternately to thereby form the insulation layer.
It is also preferable that at least one of xcfx81(resistivity)-grading or xcex5(permittivity)-grading is formed in the insulation layer. For example, a composite tape laminated with kraft paper on both sides of polyolefin in resin film is used as the insulating tape, and the ratio of the thickness of the polyolefin resin film to the total thickness of the insulating tape is changed to thereby form the grading. Not to say, the composite insulating tape used here may include an insulating tape in which the thickness of kraft paper is zero, that is, which consists of only the polyolefin resin film.
Further, when a composite tape laminated with kraft paper on both sides of polypropylene film (PPLP) is used as the insulation layer, it is suitable to make the ratio of the thickness of the polypropylene film to the total thickness of this composite tape not less than 40% and less than 90%. Particularly, it is more preferable that this ratio is set to exceed 60%.
Generally, a metal sheath (usually a lead sheath) is provided on the outer circumference of an insulation layer of a solid cable. It is also preferable to form a reinforcing tape layer on the outer circumference of this metal sheath. This reinforcing tape layer has a function to have its share against hoop stress (stress generated inside the metal sheath by oil pressure to break the metal sheath) exerted on the metal sheath to thereby reinforce the metal sheath. Therefore, it is preferable to select the material of the reinforcing type layer from the materials which can obtain a high tensile strength, for example, from polyamide, polyimide resin tape (trade name; Kevlar), etc. as well as a metal tape such as stainless steel.
As for the method of manufacturing a solid cable according to the present invention, the above-mentioned medium-viscosity insulating oil may be impregnated in a conventional method as it is. In addition, the method of manufacturing a solid cable according to the present invention comprises the steps of: impregnating an insulation layer with low-viscosity insulating oil the viscosity of which is not more than 10 centistokes (cst) at a room temperature; deoiling the insulation layer to remove the low-viscosity insulating oil; and then impregnating the insulation layer with medium-viscosity insulating oil the viscosity of which is not less than 10 centistokes (cst) and less than 500 centistokes (cst) at 60xc2x0 C. Also in this case, it is preferable that the SP value of the medium-viscosity insulating oil is within a range of xc2x11.5 of the SP value of polyolefin resin.
Further, the transmission line according to the present invention comprises a submarine-portion solid cable laid on the bottom of the sea which is constituted by the above-mentioned solid cable according to the present invention, and land-portion cables connected to both ends of the submarine-portion solid cable through oil-stop joint boxes respectively, the oil-stop joint boxes being disposed on shore portions, oil feeding tanks being connected to the land-portion cables for feeding insulating oil having medium or lower viscosity to the land-portion cables.
Here, the land-portion cables may be solid cables or OF cables (Self-Contained Oil-Filled Cables). Insulating oil the viscosity of which is medium or lower is supplied from the oil feeding tanks when the land-portion cables are solid cables, and low-viscosity insulating oil is supplied in the case of the OF cables. In addition, the above-mentioned transmission line is preferably configured in the manner that oil feeding pipes are connected to the oil-stop joint boxes at their submarine-portion solid cable sides, and the oil feeding pipes are coupled with the oil feeding tanks so as to feed medium-viscosity insulating oil from the oil feeding tanks to the submarine-portion solid cable. Further, more preferably, a check valve is provided in this oil feeding pipe so as to make the medium-viscosity insulating oil flow only toward the oil-stop joint box.